1. Field of the Invention
The present invention relates to an electrically modulatable thermal radiant source.
2. Description of Related Art
Infrared radiant sources are used in optical analysis methods as IR radiation sources, and in some other applications as heat sources. Several different types of IR sources are used for the former application, such as the "globar" source, the incandescent lamp, and the thick-film radiator. The intensity of the radiation beam emitted by the IR source can be modulated by altering the source temperature through varying the input power to the source, or alternatively, by using a mechanical beam interrupting device called a "chopper", while simultaneously keeping the source temperature as constant as possible.
When a mechanically movable chopper is used for modulating the beam, the mean time between failure of the radiant source is usually limited by the chopper mechanism life, typically lasting from one year to two. An electrically modulated source provides a much longer time between failure.
Analogous to its name, a "globar" is a glowing bar. The bar is conventionally made from a ceramic material heated with electric current. A "globar" device is typically a few millimeters thick and a few centimeters long, whereby its thermal time constant is several seconds. The "globar" is usually not modulated by varying the power input to the device. The input power is typically in the range from a few watts to a hundred watts. A variant of the "globar" device is a ceramic bar having a resistance wire wound about the bar. The thermal properties of the variant are equivalent to those of the simple "globar".
An incandescent lamp can be electrically modulated with frequencies up to tens of Hz, even up to several hundred Hz, but the glass bulb of the lamp absorbs radiation in the infrared range and blackens in the long run, whereby the output intensity of radiation delivered by the lamp decreases with time. The required input power is typically from a few watts to tens of watts.
A thick-film radiator typically comprises a thick-film resistor formed onto an alumina substrate and heated by electric current. The size of the resistor is typically in the order of a few square millimeters and it has a thickness of half a millimeter. The thermal time constant of the resistor typically is in the order of seconds and the required power input is a few watts.
Conventional production techniques used in microelectronics and micromechanics provide the ability to produce miniature size, electrically modulated radiant sources from silicon. See, for example, 1) H. Guckel and D. W. Burns, "Integrated transducers based on black-body radiation from heated polysilicon films," Transducers '85, 364-366 (Jun. 11-14, 1985); 2) Carlos H. Mastrangelo, James Hsi-Jen Yeh, and Richard S. Muller: "Electrical and optical characteristics of vacuum sealed polysilicon microlamps," IEEE Transactions on Electron Devices, 39, 6, 1363-1375 (June 1992); and 3) M. Parameswaran, A. M. Robinson, D. L. Blackburn, M. Gaitan and J. Geist, "Micromachined thermal radiation emitter from a commercial CMOS process," IEEE Electron Device Lett., 12, 2, 57-59 (1991). Such devices have a thin-film structure of polysilicon with a typical thickness of approx. one micrometer and a length of hundreds of micrometers. The width of the thin-film resistive element may vary from a few micrometers to tens of micrometers. The thermal capacity of such a silicon incandescent filament is low permitting its modulation with frequencies up to hundreds of hertzes. Pure silicon is an inferior conductor for electric current. However, by doping it with a proper dopant such as, e.g., boron or phosphorus, excellent conductivity is attained. Boron as a dopant is handicapped by the fact that its activation level is not stable, but rather, is dependent on the earlier operating temperature of the silicon incandescent filament. This causes the activation level to continually seek a new equilibrium state, which means that the resistance of the filament drifts with time, and so does the input power to the filament unless the power input is not externally stabilized. The highest impurity concentration possible in silicon with boron as dopant is approx. 5.multidot.10.sup.19 atoms/cm.sup.3. Other conventional dopants are arsenic and antimony. A problem encountered with these elements as dopants is the difficulty of achieving adequately high impurity concentrations for attaining a sufficiently high conductivity for low-voltage use.
The incandescent filament discussed in H. Guckel and D. W. Burns, "Integrated transducers based on black-body radiation from heated polysilicon films," Transducers '85, 364-366 (Jun. 11-14, 1985) is made by doping with phosphorus to achieve a sheet resistance greater than 50 .OMEGA./square. The incandescent filament is 100 .mu.m long, 20 .mu.m wide and elevated from the substrate by 1.2 .mu.m. In such a structure, the radiant power loss over the air gap to the substrate is particularly high, and a high risk of the filament adhering to the substrate is evident as the filament sags during heating.
Both boron and phosphorus doping have a problem related to the migration of dopant atoms. This manifests itself in the form of a hot zone developing at that end of the filament wherefrom the dopant atoms have migrated away. Resultingly, the emission intensity profile of the incandescent filament undergoes a gradual change, which is observed as a long-term instability.
The structure of the incandescent filament discussed in Carlos H. Mastrangelo, James Hsi-Jen Yeh, and Richard S. Muller: "Electrical and optical characteristics of vacuum sealed polysilicon microlamps," IEEE Transactions on Electron Devices, 39, 6, 1363-1375 (June 1992) comprises encapsulation under a thin-film window and placing the incandescent filament in a vacuum to avoid burn-out. Such a window cannot be wider than a few tens of micrometers, whereby the total surface area of the filament, and accordingly, its radiant output remain small. To avoid adherence of the filament, a V-groove is etched into the substrate.
The IR emitter discussed in M. Parameswaran, A. M. Robinson, D. L. Blackburn, M. Gaitan and J. Geist, "Micromachined thermal radiation emitter from a commercial CMOS process," IEEE Electron Device Lett., 12, 2, 57-59 (1991) has a size of 100 .mu.m by 100 .mu.m and uses two meandering polysilicon resistors as the heating element. Such a structure is prone to warp during heating, and large-area emitting elements cannot be manufactured by way of this concept. Though the heating element is continuous, the gas bubbles emerging during the etching phase of the substrate cause no problems as the heating element size is small in comparison with the openings about it. However, the temperature distribution pattern of this structure is not particularly good as is evident from FIG. 2 of the Parameswaran et al. article.
An incandescent filament made from doped polysilicon is associated with a characteristic temperature above which the temperature coefficient of the filament resistance turns negative, that is, allows the filament to pass more current with rising temperature. Consequently, such a component cannot be controlled by voltage, but rather, by current. Neither can such filaments be connected directly in parallel to increase the radiant source surface as the current tends to concentrate on that filament having the lowest resistance, that is, the highest temperature. Series connection on the other hand requires elevating the input voltage to a multiple of the single filament voltage. Boron doping cannot provide a satisfactorily high characteristic temperature, because a high boron impurity concentration only achieves a characteristic temperature of approx. 600.degree. C. If the operating temperature of the filament is higher than this, the filament resistance tends to drift with time.
In the embodiment according to Semiconductor International, p. 17, November 1992, the incandescent filament is made of a thin-film metallic layer. To prevent oxidization, the incandescent filament is encapsulated in a vacuum.